Zeolites are crystalline aluminosilicate compositions which are microporous and which have a three-dimensional oxide framework formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure.
One important process which uses zeolites is the separation of nitrogen from air using Pressure Swing Adsorption (PSA). The types of zeolites which are used for air separation are low silica zeolites such as those having the faujasite topology (FAU) and having a SiO2/Al2O3 of less than 3.0 and preferably about 2.0. Considerable effort has been expanded in order to develop adsorbents with improved selectivities for nitrogen versus oxygen. However, these efforts have resulted in only incremental improvements.
In view of the expense and somewhat low return of this research, it would be very beneficial to develop a method which could accurately predict the structure (especially the extra-framework cations) and adsorption isotherms of zeolites. Development of such a method requires knowing the physics of air separation in low silica zeolites. It is known from the art that the energy of adsorption is the sum of dispersion, electrostatic and induction terms. N2 has a larger quadruple moment than O2 which causes N2 to have a larger heat of adsorption than O2 on low silica zeolites. The larger adsorption heat causes low-silica zeolites to be more selective for N2 than O2. An adsorbent with a larger void volume is advantageous because it can adsorb more sorbate molecules. This allows the adsorbent to maintain selectivity at higher loadings. In spite of the simplicity of calculating adsorption energy, the state of the art in predicting structure and properties of zeolites cannot predict the relative performances IS of materials for air separation. Fairly high levels of accuracy are needed to predict the relative performance. An error of 0.1 kcal/mol in the free energy of adsorption leads to errors˜15% in loadings at room temperature. This level of accuracy is required to predict relative performance. Current methods of predicting performance do not yield results with 0.1 kcal/mol accuracy.
Applicant has developed a method which accurately predicts zeolite structure including the cation locations and further determines the adsorption isotherms for these zeolites. A key feature of the method is that it can accurately predict the location of the cations. Cation location is important because N2 has a larger quadruple moment that O2. This larger moment interacts favorably with the large electric field gradient near the cation resulting in N2 being selectively adsorbed on the cations. Accordingly, a method which can predict the number and position of exposed cations on a zeolite can determine the relative selectivity of the zeolites without synthesizing them, thus quickly narrowing potential candidates.